De-icing salt corrosion: protecting steel in Quebec

De-icing salts leave soluble chlorides on steel that attract moisture, speed up corrosion and make paint applied over them blister. In Quebec, areas exposed to road splash are often classified C4 or C5 under ISO 12944-2, and repainting requires measuring salts to ISO 8502-6 and ISO 8502-9, then washing the steel down to the limit set by the specification before applying the system.

Every winter, roads, parking structures, loading docks and plant yards receive sodium chloride, calcium chloride or magnesium chloride. Slush and splash carry those salts to beams, columns, bearings and nearby framing. This guide helps asset owners and maintenance managers in Quebec and Ontario protect exposed steel for the long term, alongside our steel structure painting page.

How do de-icing salts speed up corrosion?

Chlorides speed up corrosion in three ways: they make surface water more conductive, they prevent a protective rust layer from forming and they stay on the steel even after it dries. Contaminated steel corrodes faster and, above all, more locally.

• Hygroscopic action. Calcium and magnesium salts pull moisture from the air and keep the surface wet in dry weather, which lengthens the time of wetness.
• Pitting and pack rust. Chlorides concentrated at the bottom of pits sustain corrosion that progresses under the rust, often on bottom flanges and connections.
• Osmotic blistering. Salts left under a new film draw water through the paint and cause blisters, sometimes soon after the work.

The worst-hit areas are always the same: beam ends under expansion joints, bearings, bottom flanges, columns at grade, and any surface that holds slush.

Which ISO 12944 class applies in Quebec road and industrial settings?

The class depends on the actual exposure of each part of the structure, not on the region alone. ISO 12944-2 defines atmospheric corrosivity categories C1 to C5 and CX, and notes that de-icing salts can make an environment locally more corrosive than the general atmosphere.

For a reliable classification, the standard also provides for measuring mass loss on coupons exposed on site. Our article on ISO 12944 covers the classes, and the one on C3, C4 and C5 paint systems shows how the class drives the system.

How do you measure soluble salts before painting?

Salts are extracted from a known area to ISO 8502-6, then the conductivity of the solution is measured to ISO 8502-9. The result, expressed as a surface density of salts, is compared with the maximum limit set by the project specification or the system data sheet.

• Where to test. On the most exposed areas (beam ends, bearings, flanges), after preparation and just before application, not only on easy-to-reach surfaces.
• How many tests. The specification's sampling plan applies; if there is none, define one by member and by exposure zone.
• Other methods. Chloride-specific methods exist; SSPC-Guide 15 describes field methods. The chosen method goes into the specification, because results do not compare directly from one method to another.
• Log. Location, method, result, washing done and retest are recorded for each area.

ISO 8502-6 does not set an acceptance limit: the specification does. If it is silent, settle the question before work starts.

How do you wash contaminated steel before painting?

Wash with fresh water under pressure, test again, and repeat until the limit is met. Abrasive blasting alone is not always enough: it can drive salts deeper into pits instead of removing them.

• Pre-wash. Fresh water under pressure before blasting, to remove deposits and a large share of the salts.
• Blast, then wash again if needed. On heavily pitted steel, the wash, blast, test cycle is repeated on areas still over the limit.
• Hydro blasting. High or ultrahigh pressure water jetting to SSPC-SP WJ removes salts from pits effectively; see our hydro blasting service.
• Drying and flash rust. Washed steel must dry before painting; the tolerated flash rust level is set by the specification. See our article on flash rust.

Wash water and residue are handled under the applicable environmental rules, especially over a watercourse or storm sewer.

Which paint system protects steel exposed to chlorides?

A multi-coat system with a zinc-rich or epoxy primer, applied over blasted and decontaminated steel, is the most common answer in C4 and C5. Nominal thickness and target durability are chosen from ISO 12944-5 or the owner's specification.

• Primer. Zinc-rich primer on high-grade blasted steel, or surface-tolerant epoxy for localized maintenance.
• Intermediate. High-solids epoxy for barrier thickness.
• Topcoat. Polyurethane or polysiloxane for UV resistance and cleanability.
• Critical areas. Stripe coat on edges, welds and bolts, brushed on before each full coat.

For repainting an existing structure, our article on maintenance painting of steel structures explains when to spot repair, overcoat or remove everything. These systems are part of our protective coatings.

What precautions apply to bridges and parking structures?

On bridges and parking structures, steel is only part of the problem: concrete and its rebar face the same chlorides, and salt water follows whatever path drainage leaves it. Paint maintenance must therefore go hand in hand with maintenance of joints, drains and concrete surfaces.

• Bridges. Beam ends and bearings under leaking joints are treated first, with reinforced protection. Our bridge painting page covers containment and residue handling.
• Multi-level parking. Steel columns and beams at salted slab level, guardrails and column bases corrode first. Slabs fall under CSA S413 and suitable concrete protection, and active cracks call for crack injection.
• Seasonal washing. A fresh-water wash in spring removes winter salts and slows corrosion between two painting campaigns.

How do you plan the work and get a quote?

Start with a condition survey and a map of exposure zones, then set the corrosivity class, the salt test method, the limit and the system. Painting is scheduled in the warm season, after the spring wash, so that drying and ambient conditions are favourable.

Between campaigns, a simple routine pays off: inspect the critical areas each spring, clear debris from bearings and girder ends, check that joints and drains work, and touch up damaged paint before rust spreads. That routine costs little compared with a full repainting, and it keeps the record of each area up to date for the next campaign.

Our crews and qualified subcontractors work from LaSalle, including Montreal. Send photos of corroded areas, dimensions and the structure's location through the quote request: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Is calcium chloride more corrosive than sodium chloride?

It keeps the surface wet longer because it attracts moisture from the air, which extends the corrosion period. Both bring chlorides and require the same decontamination before painting.

Does abrasive blasting remove the salts?

Not always. On pitted steel, the abrasive can leave salts at the bottom of pits. Testing after blasting confirms it, and a water wash completes the preparation where needed.

What salt limit should you aim for?

The one set by the project specification or the system data sheet, for the chosen test method. It must be defined before work starts.

Does galvanized steel resist de-icing salts?

Better than bare steel, but zinc is consumed faster in the presence of chlorides. A duplex system, galvanizing plus paint, is often chosen in splash zones.